A Plethora of Epigenetic Minerals Reveals a Multistage Metasomatic Overprint of a Mantle Orthopyroxenite from the Udachnaya Kimberlite

Abstract

More than forty mineral species of epigenetic origin have been identified in an orthopyroxenite from the Udachnaya-East kimberlite pipe, Daldyn kimberlite field, Siberian platform. Epigenetic phases occur as: (1) Mineral inclusions in the rock-forming enstatite, (2) daughter minerals within large (up to 2 mm) crystallized melt inclusions (CMI) in the rock-forming enstatite, and (3) individual grains and intergrowths in the intergranular space of the xenolith. The studied minerals include silicates (olivine, clinopyroxene, phlogopite, tetraferriphlogopite, amphibole-supergroup minerals, serpentine-group minerals, talc), oxides (several generations of ilmenite and spinel, rutile, perovskite, rare titanates of the crichtonite, magnetoplumbite and hollandite groups), carbonates (calcite, dolomite), sulfides (pentlandite, djerfisherite, pyrrhotite), sulfate (barite), phosphates (apatite and phosphate with a suggested crystal-chemical formula Na2BaMg[PO4]2), oxyhydroxide (goethite), and hydroxyhalides (kuliginite, iowaite). The examined epigenetic minerals are interpreted to have crystallized at different time spans after the formation of the host rock. The genesis of minerals is ascribed to a series of processes metasomatically superimposed onto the orthopyroxenite, i.e., deep-seated mantle metasomatism, infiltration of a kimberlite-related melt and late post-emplacement hydrothermal alterations. The reaction of orthopyroxene with the kimberlite-related melt has led to orthopyroxene dissolution and formation of the CMI, the latter being surrounded by complex reaction zones and containing zoned olivine grains with extremely high-Mg# (up to 99) cores. This report highlights the utility of minerals present in minor volume proportions in deciphering the evolution and modification of mantle fragments sampled by kimberlitic and other deep-sourced magmas. The obtained results further imply that the whole-rock geochemical analyses of mantle-derived samples should be treated with care due to possible drastic contaminations from “hiding” minor phases of epigenetic origin.

Hydroxychloride‐Bearing Fluid Inclusions in Ultramafic Rocks From New Caledonia: Implications for Serpentinization in Saline Environments on Earth and Beyond

“…A similar situation has been observed for brucite, with incorporation of considerable ferrous iron into brucite at low fluid flux followed by oxidation of the ferrous iron into magnetite at high fluid flux (Bach et al, 2004(Bach et al, , 2006Frost et al, 2013). Kuliginite coexists with magnetite and iowaite, the latter being an oxidized hydroxychloride containing ferric iron (Bach et al, 2004;Heling & Schwarz, 1992), in Udachnaya kimberlite (Mikhailenko et al, 2018;Rezvukhin et al, 2020), suggesting that magnetite and iowaite may be the oxidized products of kuliginite. Therefore, it is proposed that kuliginite formed during serpentinization under mineral-dominated conditions may be oxidized into magnetite and iowaite under fluid-dominated conditions via the following reaction:…”

“…Kuliginite, with the ideal formula of Fe 3 Mg(OH) 6 Cl 2 , is a hydroxychloride mineral discovered and presently only reported in the Udachnaya East kimberlite pipe, Russia (Mikhailenko et al., 2018; Rezvukhin et al., 2020). It occurs along with other chlorides, calcite, and sulfates in weakly serpentinized kimberlite, which has been suggested to form during post‐emplacement hydrothermal alteration by brines (Mikhailenko et al., 2018; Rezvukhin et al., 2020).…”

“…Kuliginite, with the ideal formula of Fe 3 Mg(OH) 6 Cl 2 , is a hydroxychloride mineral discovered and presently only reported in the Udachnaya East kimberlite pipe, Russia (Mikhailenko et al., 2018; Rezvukhin et al., 2020). It occurs along with other chlorides, calcite, and sulfates in weakly serpentinized kimberlite, which has been suggested to form during post‐emplacement hydrothermal alteration by brines (Mikhailenko et al., 2018; Rezvukhin et al., 2020). It was previously considered that kuliginite may only be a localized mineral, as the Udachnaya kimberlite is unique by the presence of abundant chlorides in some units that are quite uncommon in other kimberlite pipes (Kamenetsky et al., 2014; Mikhailenko et al., 2018).…”

Micro‐Raman study of crichtonite group minerals enclosed into mantle garnet

“…[40,45] Enstatite-hosted CGM inclusions have been recently recognized in an orthopyroxenite from Udachnaya. [41,75] The CGM-bearing samples from Siberia are known to have been equilibrated at pressures of 2.5-4.2 GPa and temperatures of 650 C-850 C. [22,24,[39][40][41]73] Mantle CGM are considered to be a valuable reservoir of a large number of high field strength, large ion lithophile, and REE with their abundance up to 100 chondritic units. [10,16,26,76] This fact together with CGM stability at high pressures and temperatures makes them potentially important repository for rock-forming mantle silicate-incompatible trace elements, together with well-known metasomatic minerals-phlogopite, amphibole, and carbonates-and, thus, provides clues to fluid and magma generation at upper-mantle conditions and metasomatism of the depleted lithospheric mantle.…”

Editorial for Special Issue “Minerals of Kimberlites: An Insight into Petrogenesis and the Diamond Potential of Deep Mantle Magmas”

“…Rezvukhin et al [8] provides a detailed study of mineralogy of the orthopyroxenite xenolith from the Udachnaya kimberlite pipe. They have identified more than forty epigenetic minerals in the inclusions in rock-forming enstatite and in the intergranular space of the xenolith.…”